Many mobile electronic systems require mobile power sources. These systems have power requirements that range from milliwatts to hundreds of kilowatts, and include systems from small embedded sensors to motor vehicle power systems. Currently, these power requirements are addressed by a range of technologies that includes electro-chemical batteries, photovoltaic cells, thermoelectric generators, fuel cells and internal combustion engines. These solutions, however, are oftentimes insufficient in a number of important aspects, including a low energy density (electro-chemical batteries), a dependency on an external primary energy source (photovoltaic and thermoelectric generators), high cost and complexity (fuel cells), low efficiency (internal combustion engines) and an inability to scale to small systems (diesel and spark ignition internal combustion engines.)
Homogenous Charge Combustion Ignition (HCCI) has been recognized as a new combustion mode for internal combustion engines. HCCI relies upon a lean and well-mixed air-fuel mixture that is compressed. A resulting spontaneous burn produces a flameless energy release in a large zone almost simultaneously. This operation is very different from the spark/gasoline burn or the compression/diesel burn. HCCI can thus be a basis for an efficient engine, like a diesel engine, but without the NOx or particulate emissions of diesel. In rotating, fixed compression engines, HCCI, however, has resulted in increased emissions of unburned hydrocarbons. In addition, the development of these engines has been hindered by the requirement for complex control mechanisms.
A free-piston Microcombustion Engine/Generator is known to use HCCI on a small scale to achieve electrical power generation on the order of 10 W from liquid fuels.
Implementation of HCCI on a small scale that uses pistons in a chamber severely limits the efficiency, and possibly the viability, of a microcombustion engine due to “blowby” of the air/fuel mixture during compression and of exhaust products during the power stroke of the engine. More importantly, fabricating and operating pistons in cylinders at such small scale leads to significant, if not insurmountable, practical difficulties.
It has been shown that a gap which is 1/1000th of a diameter of the piston itself is large enough to reduce engine efficiency by a factor of two or more; and limit the actual compression ratio, because the piston is compressing a fluid into a cylinder while the fluid is simultaneously leaking out around the piston walls. Further, it is impractical to implement such tight manufacturing tolerances using currently known micromachining practices.
The issues associated with pistons in cylinders, however, are not limited to fabrication technology and its associated minimum tolerances. Even if a piston and cylinder could be fabricated with closely matching sizes, these dimensions would change in situ as a result of uneven heat flow in the system generated by high temperature combustion that occurs at the piston head. Even with material of matched thermal coefficients of expansion, the piston would tend to become instantaneously hotter than the surrounding cylinder, and, if the separation distance were too small, would tend to bind in the cylinder. At the same time, increasing the separation distance between the piston and cylinder increases blowby and, therefore, limits the attainable compression. Limited compression can compromise the performance or, in some cases, prevent the occurrence of HCCI, thus rendering the design inoperable.